You should open a new question if your problem is different from the existing question. If you're aware of similar existing questions, it's a good idea to mention them in your own question and briefly explain why they don't help you (e.g. highlight a different symptom).
There's an edge case where the existing question is a troubleshooting question with too little detail to get helpful answers. If you're going to stick around and answer requests for further information, and the existing question looks abandoned, I think it's best to ask a new question, and mention that the existing question may or may not be about the same problem but it has too little information to tell. Furthermore, such a situation is an indication that the older question should be closed as “not a real question” (if it is too vague or incomplete to be meaningfully answered).